Domain propagation devices are well known in the art. In most such devices, a reverse-magnetized domain, having spaced-apart leading and trailing domain walls, is moved controllably in a channel structured to prevent lateral motion of the domain. The Bell System Technical Journal (BSTJ), Volume XLVI, No. 8, Oct. 1967, at page 1,901 et seq., on the other hand, describes a domain which is (self) bounded by a single domain wall in the plane of the sheet and is thus free to move in that plane. Movement of a domain in the latter case is in response to a magnetic field (gradient) which displaces the domain in the absence of uncontrolled expansion thereof.
A typical magnetic sheet in which single wall domains are moved comprises, for example, a rare earth orthoferrite or a strontium or barium ferrite. The domains assume the shape of circles in the plane of a sheet of these materials. The sheets are characterized by a preferred direction of magnetization normal to the sheet, the magnetization in a first direction along that normal being considered negative (-) and the magnetization in a second direction being considered positive (+) A single wall domain in such a sheet may be visualized as a circle representing the encompassing single wall of the domain.
There are a variety of techniques for moving single wall domains. One technique comprises offset conductor loops pulsed in sequence to attract domains to next consecutive positions. This technique permits the greatest degree of control over individual domains. But the current-carrying requirements of such conductors make it difficult to realize the minute dimensions required to manipulate, for example, domains of the order of microns.
Another technique for moving signal wall domains employs a structured magnetically soft overlay on the sheet in which single wall domains are moved. Such an implementation is disclosed in copending application Ser. No. 732,705 now U.S. Pat. No. 3,534,347, filed May 28, 1968 for A. H. Bobeck. Magnetic poles move in the overlay in response to reorienting in-plane fields. The poles attract domains along a predictable path determined by the overlay pattern and the consecutive orientations of the field. This technique has the virtue that the overlay has to meet no current-carrying requirements and so can be adapted for manipulating domains of minute size. The technique also permits the movement of all domains in a sheet without discrete wiring connections.
A propagation technique employing such an overlay is clearly attractive for recirculating type memories, such as disc files, where information is moved constantly and the read and write operations are carried out at a common location. This type of organization is presently realized in accordance with prior art electromechanical techniques which provide economy and reliablity by reducing the number of detection and input circuits.
An object of this invention is to provide a new and novel detection arrangement for single wall domain propagation arrangements employing a magnetically soft overlay geometry.